Structures Found on Wave-cut Benches in the Southern Miura Peninsula
Location and geology of the Miura Peninsula
The Miura Peninsula is located about 50 kilometers south-southwest of Tokyo. The eastern side of Miura Peninsula borders on Tokyo Bay and the western side on Sagami Bay. Sagami Bay has the Sagami Trough where the Philippine Sea Plate is subducting underneath the North American Plate. The Izu Peninsula on the west of Sagami Bay is on the northern end of the Izu-Bonin (Ogasawara) Arc. This island arc coming from the south reached the Honshu arc about 15 million years ago because the Philippine Sea Plate has been moving northward, and the Izu Peninsula collided with the Honshu Island 1.5 million years ago (see Formation history of the Japanese Islands [p.4]). The boundary zone between the Izu Peninsula and the Honshu Island is, therefore, a collision zone. The Miura Peninsula is in the extended area of the collision zone. Because accretionary prisms have developed since 15 million years ago, accreted deposits derived from sediments on the forward edge of the Izu-Bonin Arc are found in the region including the Miura Peninsula and the Boso Peninsula.
Figure 1: Location of the Miura Peninsula
The Miura Peninsula consists of hills in the northern part and marine terraces in the southern part. Thick Late Cenozoic marine deposits are exposed in zones stretching almost east to west. Five major active faults cross the peninsula in the WNW-ESE direction. Of these faults, four faults are the borders of Neogene formations. The oldest formation group is the Hayama Group (Early Miocene) distributed in two NW-SE uplift zones in the middle part of peninsula. The formations in the group are deposits of accretionary prism extremely deformed, consisting mainly of siltstone and alternating beds of silt and tuffaceous sand. The formations also include small blocks of serpentinite, alkali basalt and dolerite. The Zushi Formation containing siltstone (Late Miocene to Late Pliocene) is distributed between the two zones and on the north side of the Hayama Group. The Ikego Formation comprising alternating beds of pyroclastic rock/tuffaceous sandstone and siltstone (Late Miocene to Late Pliocene) is on the north side of the Zushi Formation. These sedimentary rocks are derived from forearc basin deposits. Both of the formations evenly dip northeastward.
Figure 2: Distribution of formations in the Miura
Peninsula (rough map)
(Based on Ogawa et al., 2003 and Editorial committee of Geology of Japan [Kanto Region], 1986)
In the southern part of peninsula, the Miyata Formation
(Pleistocene) is on the south side of the Hayama Group, which is
composed of silt, sand, and gravel. Late Miocene to Early Pliocene
formations, the Misaki Formation and the Hatsuse Formation, are to the
south of the Miyata Formation. These Neogene formations were deformed
by many reverse faults and folds, different from the Zushi and Ikego
Formations evenly dipping northeastward.
The Misaki Formation comprises alternating beds of silt, basaltic/scoria tuff and tuffaceous sand. An abundance of small faults cut the alternating beds into many sections. The siltstone is massive and includes trace fossils. Basaltic gravel, which is larger than several centimeters in diameter and less-wearing, is found in some of the scoria tuff and tuffaceous sand, suggesting that there was a volcano that spouted out the scoria near the peninsula. Slump deposits developed in some beds. The Misaki Formation deposited in a lower middle bathyal zone or abyssal zone.
The Hatsuse Formation contains layers of redeposited pyroclastic rock or coarse-grained tuffaceous sand with well-developed cross-bedding. The age of the formation is considered as the same as that of the Misaki Formation, but the Hatsuse Formation formed in the shallow sea. However, the different opinion that the formation deposited in a bathyal zone based on the analysis of nannofossils was also proposed (Kanie, 1999).
The Zushi and Ikego Formations in the north part and the Misaki and Hatsuse Formations in the south part are conventionally grouped into the Miura Group. However, Takahashi (2008) classified the Zushi and Ikego Formations into the Awa Group as forearc basin deposits and the Misaki and Hatsuse Formations into the Miura Group as deposits of the trench or the landward slope of trench. Neogene formations correlated with the formations in the Miura Peninsula are distributed in the Boso Peninsula on the other side of Tokyo Bay.
(Based on Takahashi, 2008 and Editorial committee of Geology of Japan [Kanto Region], 1986)
This topic focuses on the Misaki Formation.